Various surgical procedures require that a region of tissue be positioned and manipulated. Two types of devices for holding and manipulating objects are clamps and vacuum devices.
Clamps and forceps apply opposing forces from two directions. However, access to two sides (e.g., front and back) of a piece of tissue is not always available. When only one side of the tissue is available, a portion of the tissue must be "pinched" between the jaws of the clamp. This can be traumatic to the tissue.
Clamps can easily apply excessive forces to local areas. For example, in vascular surgery, excessive clamping force can collapse a blood vessel. In such a collapse, the tubular vessel is flattened, stopping the flow of blood and potentially damaging the vessel tissue.
Vacuum devices present an alternative to clamps and forceps. The vacuum device is held adjacent to a surface to create an enclosed volume. Vacuum may be drawn in the enclosed volume through a port. The enclosed volume is otherwise sealed from its surroundings. Forces and displacements applied to the vacuum device are communicated to the surface via the suction pressures.
However, standard vacuum cups designed for use on rigid surfaces do not work as well on surfaces of flexible materials such as human tissues. When force is applied to soft tissue, the tissue deforms. In some casesm the deformation is linear and elastic and easily quantified. More often with biological tissues, the deformation is highly nonlinear, and may include a time-dependent, visco-elastic component. For example, human tissue exhibits both elastic and visco-elastic properties. As a result the deformation of human tissue depends on both the force currently applied and also on the cumulative past history of forces applied to the tissue. When forces are applied to human tissue for extended periods, the tissue typically undergoes large deformations and the seal with a standard vacuum cup can be easily lost.
To avoid losing the seal with flexible human tissues, a deeped vacuum cup has been developed and is currently used for childbirth extraction. For example, in U.S. Pat. No. 5281229 Neward discloses an obstetrical vacuum extractor cup. In cups such as these, the deep walls of the cup enable a seal to maintained even with highly deformed tissue. The cup is axisymmetric; the deep walls have a circular cross section prevent buckling and collapse of the cup when a vacuum is drawn.
While such a vacuum is drawn within these deepened cups, the cups can impose large forces and/or deformations to the tissue. These forces to the tissue can be significant even when no overall net force is applied to the cup.
For example, when no force is applied to the handle of a deepened cup, calculation of a force balance shows that no net force is applied from the cup to the tissue. (In this calculation, the small weight of the cup is neglected). To achieve the zero net force on the tissue, the pulling force on the tissue caused by the vacuum in the interior of the cup is balanced by an equal pushing force applied to the tissue by the rim of the cup. These forces which act on the tissue when no net external force is applied to the vacuum device are defined herein as residual forces.
The residual forces can cause undesirable deformations and possible trauma to the tissue. Obstetrical cups create a large deformation called an "artificial caput succedaneum" on a portion of the fetal head. This condition requires a considerably long period for a complete cure and sometimes leaves a permanent scar on the head of the fetus. Additionally, the gross distortion of the infant scalp causes bruising. This bruising elevates the blood levels of bilirubin, which the immature infant liver is not yet capable of removing, and jaundice can result.
Furthermore, the effectiveness of the vacuum device diminishes after the shape of the tissue conforms to the shape of the rigid cup. After the tissue has deformed to the cup shape, any additional deformation can result in loss of the seal between the cup and the tissue.
An alternative to deepened cups are cups of a very small diameter. With smaller diameter cups, the applied forces are much smaller. Furthermore, the elastic and viscoelastic effects are mitigated by the small length scales. Small length scales mean that the tissue must be deformed to a smaller radius of curvature to separate from the cup. Correspondingly higher stresses are encountered; the materials appear effectively stiffer as the length scales decrease. However, the small forces that these small cups can deliver severely limit their usefulness for securely holding tissue.
Therefore, there is a need for a system for holding tissue during surgery which can be easily applied from one side of the tissue, which can securely hold the tissue without excessive deformation or damage, and in which the residual forces are very small.